Studies on the Actomyosin ATPase and the Role of the Alkali Light Chains

Myosin isoenzymes, highly enriched in either alkali 1 or alkali 2 light chains have been prepared by light chain exchange in 4. 7 M ammonium chloride, under conditions where there is minimal loss of ATPase activity. While the actin-activated ATPase measurements were complicated by a biphasic dependence on actin concentration, the two myosin isoenzymes behaved in a similar manner; at a variety of ionic strength conditions their maximum rates of ATP hydrolysis were nearly identical. Furthermore, under conditions where their Km values could be reliably determined, their apparent affinities for actin in the presence of ATP did not differ greatly. These results suggest that the presence of a particular alkali light chain does not influence the maximum rate of ATP turnover by actomyosin under ionic strength conditions approximating physiological.